Biodeterioration of stone has been the subject of numerous studies in recent years. This report presents conclusive evidence obtained with scanning electron microscopy (SEM) of fungal attack on dolomitic and calcitic stone. The attack by Trichothecium sp is rapid, deep into the stone, and has a characteristic appearance. This study originated from a program to elucidate the contributions of microorganisms to the deterioration of specific regions of the Spanish Apse of Fuentiduena. This Apse was constructed from a soft yellow dolomitic limestone. Two areas, where water run-off periodically inundates the stone, have severe decay. Large populations of microorganisms are abundant in these areas. Samples of microorganisms from the Apse were obtained and cultured in the laboratory. The microorganisms, a Trichothecium sp. Fungus, a Chlorococcales alga, and a cyanobacterium, were cultured on: 1) single crystals of dolomite; 2) single crystals of calcite; and 3) thin sections prepared from the limestone of the Apse. The extent and type of deterioration obtained was studied after five weeks in culture. Optical, epi-fluorescent, and scanning electron microscopy were employed. Implications of the contribution of biodeterioration to the overall decay of the limestone of the Apse are discussed.